This thread is for questions specifically relating to the Science Buddies wind tunnel design. I will answer your questions to the best of my ability, and since I designed and built it, I should be able to provide some insight.

Good luck on your wind tunnels, and feel free to post any of your questions/concerns/comments!

The following is an email conversation between myself (designer of the wind tunnel) and a man named Bill who was using the wind tunnel design. I have posted it here for the benefit of those who may find it useful. Bill's emails are in boldface to distinguish them from my own:

Hello AerospaceGuy,

My question is in regards to the wind tunnel that you built. Were you ever able to determine the actual wind speeds achieved in the test chamber. I need to build a wind tunnel to test some projects for our R&D facility that can achieve wind speeds between 5-30 MPH and I was hoping that your design would fit the bill.

Regards,

-Bill

Hi again Bill,

There is a way to mathematically determine the speeds in the test chamber. You can use the CFM (cubic feet per minute) rating of the drive section fan to determine the wind speed using this simple fluid dynamics formula:

Wind speed = (volumetric flow rate)/(flow cross section area)

In equation form: v = Q/A, where v is wind speed, Q is volumetric flow rate, and A is cross section area.

The volumetric flow rate is the CFM rating, and the flow cross section area is just 1ft^2 since the test section is 1ft by 1ft. So, you’ll end up with a number measured in feet/minute. Then just use arithmetic to convert ft/m into MPH.

The fan that I recommend using for the tunnel is between 1600CFM and 2000CFM. I did the calculations, and so my recommendation says the same thing as getting a fan between 18.18MPH and 22.73MPH. Since your range is between 5-30MPH, it would seem that the entire recommended range of fans is within your achievement domain. Also, in my design I have you use a variable voltage switch to be able to adjust the fan’s speed, if that is helpful to you. If you need to do that, I can talk you through how to use a voltage ratio to calculate the wind speed adjustment that you need to make.

As for directly measuring the wind speed in the tunnel (if you want to take data instead of calculating what the wind speed should be), then you should use a device called an anemometer, and mount that in the tunnel instead of the wind speed monitor that I have in my design. This could be more useful and more accurate than calculation, because the tunnel is never going to be perfect so your calculation will always be a little off due to equipment error. Remember, this is a high school research-level tunnel.

Did I answer your questions in a way that is helpful to you?

-AerospaceGuy

Thank you AerospaceGuy!

I would like to start by saying what a pleasure it is to deal with someone who knows what they are talking about…

I ran a guesstamation yesterday and determined that a 1600 cfm (available at the local home depot) would require approximately 6” x 6” cross sectional area in the test section to achieve 30 MPH.

I have 1600 cfm and need 2640 fpm

1600/2640 = ~.6

So I built a diffusion chamber with a 6” x 6” orifice and measured the wind speed with a small handheld anemometer that my boss had kicking around his basement. I was able to achieve 22 MPH. I then proceeded to build the test section that is 3” x 6” with a small adapter to the existing orifice. This was able to achieve 31 MPH.

(in hindsight 6” x 6” is ¼ not ½ of 12” x 12”)

Going back to try to understand the math. I know it is a simple formula but I am having trouble wrapping my head around it for some reason.

Wind speed = (volumetric flow rate)/(flow cross section area)

Solve for cross section =>

Cross section = volumetric flow rate / wind speed

Cross section = 1600 CFM / 2640 FPM

.6 ft^2 = 86.4 in^2

So it should have been about 9” x 9”? But it worked out better that I erred on the small side because I ended up having to make a restrictor.

One more question:

Does the collection chamber have any effect on the wind speeds or does it just serve to direct air from the settling chamber to the test chamber?

Thank you so much for the help and I can’t wait to poke around the website to see what else I can find to play with.

Regards,

-Bill

Hello again,

Your math looks good. Glad you could come up with a test section that works for you!

Your question is a very good one, by the way. The collection chamber does in fact serve to increase wind speed in the test section. This is part of what I meant about the differences between the math and actual data you would collect: the collection chamber is much more voluminous than the test section so that a large volume of air is collected and compressed to fit into the test section volume. By the continuity and Bernoulli equations, the effect produced is that the wind speed in the test section increases without the need of power. In other words, the fan draws air into the tunnel, which speeds up as it is compressed into a smaller volume.

I strongly encourage you to perform some research using the sources cited in my “Bibliography and Credits” section (http://www.sciencebuddies.org/science-f ... aphy.shtml). This will help you better understand the effects of the collection chamber than if I were to just try to explain it over email.

A few years ago I was looking at a website http://aerorocket.com/index.html for subsonic wind tunnel designs. While there I noticed that they also had supersonic wind tunnels in the form of shock tubes. From what I understand here is how they worked. A small model was put into a narrow tube and connected to electronic sensors for data gathering. The tube was connected to an air hose which was in turn hooked up to an air compressor. A thin metal membrane separated the tube (test chamber) from the hose. The metal membrane had scratches placed so that when a predetermined pressure was reached it would predictable rupture sending high pressure and high velocity air into the tube (test chamber) that would generate supersonic speeds for a very brief time. When I recently visited their web site and looked at the supersonic wind tunnel it had changed http://aerorocket.com/offer.html#anchor9 (scroll towards the bottom). The site says that it is a blow down tunnel that generates up to Mach 3 wind speeds for about 4 seconds.

I was wondering how the blow down tunnel works? Is it feasible to make either one and what materials/methods would be used?

I have started building the wind tunnel and have a question concerning the threaded rod that goes from the Vernier sensor to the airfoil in the test section. I purchased a #6-32 threaded rod which screws into the sensor. This rod seems very flexible. The project recommends a rod that does not respond to just the airflow in the tunnel but this rods seems like it will bend with little air flow. Is this the threaded rod you have used?

Hello--I will respond first to epwaotl, and then to macking in the next post.

Your question is very interesting, epwaotl. Unfortunately, supersonic miniature wind tunnels are not in my area of knowledge, so I will refer you to other sources.

I would recommend talking to the people at AeroRocket to ask about the blow down tunnel, but AeroRocket is a business and only provides information for sale. You will definitely find the information you need from that web site. However, if you are not able and/or willing to pay for it, I recommend the following sources:

I also recommend that you re-post this question on this Ask an Expert forum. We have experts who are more familiar with supersonic tunnel testing than I am who can help you with your question. Simply choose your appropriate grade level and post this question in the Physical Sciences forum for that level.

Your post says that your threaded rod will bend with only a fraction of the airflow necessary for testing, so this leads me to believe that the threaded rod you have is made of too soft a material. You will need a sturdy rod made out of something durable and resistant to bending, like steel.

Did you purchase your rod at a hardware store? I found mine at Lowe's, and I also recommend those from The Home Depot. If you did get it from a hardware store, I suggest that you bring it back and talk to a specialist from the department where you found it. Ask him/her why your rod is so flexible, and where you can find stronger ones.

There is another possible solution to your problem: it could be that your lift sensor is so far away from where the model will be in the test chamber that it will easily bend. If you are familiar with the physics concept of torque, you know that the farther away a source of force is on a rod from its fulcrum (in other words, the farther away the model is from the point at which the rod screws into the sensor), the greater the torque exerted on the rod, and therefore the higher the likelihood that the rod will bend or break.

If you look at Figure 4C from the wind tunnel guide (http://www.sciencebuddies.org/science-fair-projects/wind-tunnel-section4.shtml), you'll notice that my vertical rod is rather long, about a foot. This is a good maximum distance, and you shouldn't have your rod extending any longer than that. In fact, it's best if your rod is shorter, if you can mount your sensor closer to the test section. You have to make sure that the rod is firmly in place, and NEVER touches the rim of the hole through which it is inserted into the test section, because if it does, that will become the new fulcrum, throwing off the force sensor's readings.

Does this information make sense? I think that your problem lies mostly in the material of your rod--it sounds to me, from your description, that it's very flimsy and weak, and should be replaced by a stronger rod. Please let me know if you have further questions, and if my help leads you to solve your problem.

Should the handheld anemometer be mounted inside the tunnel off-center like the volt meter was? I thought at first I would mount it half in the tunnel and half out so I could see the digital readout easier but then thought this might disrupt the airflow. If I need to mount it completely inside the tunnel, do you think I will be able to see the readout clearly?

Very good questions. First off, you should know that the airflow behind (downwind) of the model is very important even though it's already passed over the model. This wind must be kept laminar (smoothly flowing) to ensure that no turbulence is created ahead (upwind) of the model. With this in mind, the wind speed sensor is placed offset from the center of the diffuser so that its disturbance of airflow is minimized. As I said in the writeup (http://www.sciencebuddies.org/science-f ... ion2.shtml), nearly every professionally-produced high school wind tunnel includes a wind speed monitor that blocks the airflow, and they are usually mounted off to the side, as mine is. For an example, take a look at this picture of a professionally-produced high school wind tunnel, shot down the diffuser from the test section (in other words, this is what you would see if you had a camera on the back of an airplane model being tested in this tunnel): http://s495.photobucket.com/albums/rr312/JustinSpahn/Wind%20Tunnel%20Aeolus/?action=view&current=P9030074.jpg&newest=1.

Secondly, you can mount your sensor half in and half out of the wind tunnel--just make sure that you have enough room between the tunnel wall and the spinning part of the sensor for air to flow between. Your sensor may not be very long, but you have to make sure that it extends into the tunnel as much as possible--being too close to a wall will cause an irregular airflow and mess up your measurements. So, what you want to do is mount it so that it's not too close to the walls, but also so that it's not in the center of the airflow.

All you have to do to mount the sensor is make sure that you totally plug the hole through which you insert the sensor--use glue, construction sealant, etc. You don't want your airflow disturbed by a leak!

I am now putting the sections together and, using the small digital anemometer, am getting 17 mph wind speed. I was hoping to hit 20 mph. Any suggestions before I finish putting it all together?

The small digital anemometer works well. I am using a very strong magnet outside the tunnel and two inside the tunnel to hold it in place. This way I did not have to cut an opening in the tunnel wall and the rotor portion of the anemometer is not right next to the inside wall.